Nanoparticles catalytic application

The identification of structure sensitivity would be both impossible and useless if there did not exist reproducible recipes able to generate metal nanoparticles on a small scale and under controlled conditions, that is, with narrow size and/or shape distribution onto supports. Metal nanoparticles of controlled size, shape, and structure are attractive not only for catalytic applications, but are important, for example in optics, data storage, or electronics (c.f. Chapter 5). In order not to anticipate other chapters of this book (esp. Chapter 2), remarks will therefore be confined to few examples. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Alkene hydrogenation is a common field of catalytic application for metal nanoparticles. Various approaches have been utilized to obtain stable and active nanocatalysts in hydrogenation reactions. The main approaches are described in the following sections, and are classified according to the stabilizing mode retained for the nanoparticles. 

The major disadvantages of colloidal catalysts studied so far can be attributed to problems in controlling the metal colloid formation (control of particle size, particle size distribution, structure of metal colloids) and stabilization of the prepared particles, which are not yet completely solved. But it is exactly the stability of the nanoparticles, that is decisive for long-term usage during catalytic processes. Moreover for catalytic application, it is extremely important to preserve the large surface of such colloidal systems. 

The continued development of new single-source molecular precursors should lead to increasingly complex mixed-element oxides with novel properties. Continued work with grafting methods will provide access to novel surface structures that may prove useful for catalytic applications. Use of molecular precursors for the generation of metal nanoparticles supported on various oxide supports is another area that shows promise. We expect that the thermolytic molecular precursor methods outlined here will contribute significantly to the development of new generations of advanced materials with tailored properties, and that it will continue to provide access to catalytic materials with improved performance. 

Figure 9 Examples of novel materials with potential catalytic applications. From left to right and top to bottom, these pictures represent (a) Ag nanowires. (Reprinted with permission from Ref 63. 2002 American Chemical Society) (h)Ag nanoparticles. (Reprinted with permission from Y. Sim and Y. Xia, Science, 2002, 298, 2176. 2002 AAAS (www.sciencemag.org)) (c) zeolite monolith. (Ref. 67. Reproduced hy permission of Kluwer Academic/Plenum Publishers) (d) zeolite coatings on stainless steel grids. (Ref 68. Reproducedby permission of Wiley-VCH) (e) arrays of Pt nano lithography-made particles on Si02. (Ref. 70. Reproduced by permission of Kluwer Academic/Plenum Publishers) and (f) Ag nanoparticles vapor deposited on an AI2O3 thin film ...

Niobium oxide (niobia) is an active catalyst, and can be used as a support for metal nanoparticles or oxides, and it can serve as a promoter in some reactions ([43 5] and references therein). Catalytic applications of niobia include the Fischer-Tropsch synthesis, oxidative dehydrogenation of alkanes, and oxidative coupling of methane. Studies on high-surface-area niobium oxides are complicated by a high degree of complexity because several stable structures (NbO, NbO and Nb O ) exist and the resulting surfaces of high-surface-area niobium oxides are not simple truncations of bulk niobia structures. This is even more so for supported metal oxides when two-dimensional thin films of niobium oxide partially cover a support oxide (Al Oj, SiOj, ZrOj, TiOj, [43]). Nb Oj was also used as a support for V, Cr, Re, Mo, and W oxide overlayers [45, 46]. 

Molybdenum sulfide nanoparticles in the size-range 3-10 nm have been synthesized in mild conditions using a microemulsion-based route. The reverse microemulsion phase, AOT/ n-heptane/ water, was first characterized by Transmission Electron Microscopy (TEM) of Freeze Fractures (FF) obtained via High Pressure Freezing (HPF) as well as Dynamic Light Scattering (DLS). The impacts of various parameters such as water-to-surfactant molar ratio w and the addition of a nonionic cosurfactant were then studied. The reverse microemulsion phase was further used to tailor the size of MoSx nanoparticles. The mean particle size obtained by this method makes those particles particularly interesting for further catalytic applications. 

The microemulsion-based route has already provided encouraging results, for instance in the synthesis of ZnS or CdS nanoparticles [4]. Their success is due to the easiness of finding simple cationic forms for Zn and Cd, which is of course much more difficult in the case of molybdenum. The main idea developed by Pileni and coworkers [3], which consists in forcing the supersaturation of the aqueous medium by using functionalized surfactants like (AOT)2Zn instead of (AOT)Na, cannot be applied to molybdenum either. An attempt to apply the microemulsion-based route to molybdenum sulfide nanoparticles has been carried out by Boakye et al. [2], but sizes obtained were in the size range 10-30 nm, which is still too large for catalytic applications to hydrotreating reactions. 

The objective of this study is thus to prepare MoS, nanoparticles by a microemulsion-based route in the size range 3-10 nm for catalytic applications, the particle size being directly monitored by the reverse microemulsion phase. 

In this study we showed that the control of the nucleation-growth of MoSx nanoparticles could be directly monitored via the microemulsion phase. Furthermore, the mean particle size obtained by the reverse microemulsion-based route is in the range 4-8 nm, which makes those particles particularly interesting for further catalytic applications. 

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